Ship traffic through the Strait of Hormuz fell to its lowest level since May last week, shipping data showed Monday, as Iran said it would draw a maritime "exclusion zone" through which vessels could not pass without Tehran's permission.

An average of 10 commodity ships a day transited the strait over the past 10 days, according to analytics firm Kpler, down from more than 15 on Friday and nearly 13 on Saturday. Only two vessels crossed the strait Saturday and six on Sunday, most hugging the Iranian coast. No very large crude carrier has exited the strait since Wednesday.

The traffic drop followed U.S. Central Command's strike Saturday on three Iranian oil tankers — Downy, Stark I and Kylo, also known as Noxen — and a retaliatory Iranian salvo against U.S. warships and three tankers Tehran said had used unauthorized routes. Roughly 20 percent of the world's oil and gas supplies moved through the chokepoint before the war.

Brent crude briefly touched nearly $97 a barrel early Monday, extending an 8 percent gain the prior week. Regular gasoline averaged $4.14 a gallon heading into Labor Day weekend, about a dollar higher than a year earlier and above the previous holiday record of $3.82 set in 2012.

The zone

Mohsen Rezaei, secretary of Iran's Supreme National Security Council, said the exclusion zone would be announced in the coming days and would run from the American naval blockade line into the Persian Gulf.

"This zone will start from the US Navy’s blockade line and include areas of the Persian Gulf," Rezaei told Iranian state media, according to Al Jazeera. Any ship entering the area, he said, would be added to Tehran's sanctions list.

Rezaei said Iran and Oman had agreed on new maps for a corridor that "lies in Iranian and Omani waters and in which Iran will have management" and "should be signed in the coming days". He said Tehran would "only commit to the Strait of Hormuz being open when they [the Americans] stop the sabotage, threats and attacks on Iran."

The U.S. answer

Admiral Brad Cooper, the U.S. Central Command chief, cast Saturday's tanker strikes as a formula. Cooper said, "If you shoot at two of our ships, we will impose an even higher economic cost — taking out three of yours." The command reported permanently disabling the three Iranian tankers in the Persian Gulf and Gulf of Oman.

On Sunday, CENTCOM spokesman Captain Tim Hawkins rejected an Iranian claim carried in local media that a U.S. vessel had tried to force its way into a restricted area and been fired on. Hawkins called the account "a total lie".

The rhetoric matched the maritime escalation. Iran's parliamentary speaker, in remarks reported by CBS News, said "The Americans must have understood that the era of proportionate responses has come to an end" and warned that "any aggression against Iran's interests will receive a faster, more intense and more painful response."

Insurers and owners

Maritime intelligence firm Marisks described Saturday's exchange as a "major escalation in the maritime conflict." The firm said, "Commercial tankers are now being deliberately used as instruments of reciprocal economic pressure, substantially weakening the previous distinction between military confrontation and commercial shipping." Marisks rated risk as extreme for Iranian-linked vessels and materially elevated for U.S.-linked or U.S.-escorted shipping.

The United Kingdom Maritime Trade Operations has logged 27 projectile strike incidents in the Hormuz region since July 6, with vessels damaged throughout the waterway. The 60-day memorandum of understanding that had suspended hostilities between Washington and Tehran expired last month after disputes over control of the strait, according to Al Jazeera.

What is unclear

Rezaei has not published coordinates for the proposed zone or specified what being placed on Iran's sanctions list would mean for a foreign-flagged vessel. Al Jazeera said there had been no immediate U.S. or wider international response specifically to the announcement, and no counter-declaration accompanied CENTCOM's weekend statements. Iranian state media had not offered a legal basis for the new restriction beyond Rezaei's oral description.

Kpler data showed one very large crude carrier and three bulk carriers entered the strait on Sunday. Whether they exit — and whether the Oman-brokered maps Rezaei described are signed in the coming days — will test how far Tehran intends to press its claim.